Use `objc2` instead of `objc`

[madsmtm]

Replace objc with my fork, objc2.

This crate contains many improvements, in particular:

• Better tools for protecting against UB (see first commit)
• You no longer have to worry about using bool in msg_send!.
• The msg_send_id! macro greatly helps with follow memory management rules (similar to Objective-C ARC), potentially also alleviating a lot of the need for autoreleasepools.
• The extern_protocol! macro is used in place of foreign_obj_type!

Notable breaking changes:

• objc2::Message differs from objc::Message, so message sending is not compatible

• NSUInteger is now usize instead of u32/u64 depending on platform (equivalently for NSInteger)

• There is no longer three types for each class (e.g. MTLFoo, Foo and FooRef); these have been combined into one: Foo. Pointers to this can be used across FFI boundaries, and objc2::rc::Id<Foo, Shared> can be used as a strong pointer. In table form:

  Previously	Now
  *mut MTLFoo	*mut Foo
  &FooRef	&Foo
  Foo	Id<Foo, Shared>

When done, this should resolve the following issues:

• Fixes https://github.com/gfx-rs/metal-rs/issues/5 now
• Later: https://github.com/gfx-rs/metal-rs/issues/222
• Later: https://github.com/gfx-rs/metal-rs/issues/218
• Later: https://github.com/gfx-rs/metal-rs/issues/128
• Later: https://github.com/gfx-rs/metal-rs/issues/86
• Later: https://github.com/gfx-rs/metal-rs/issues/99
• Later: https://github.com/gfx-rs/metal-rs/issues/225
• Later: https://github.com/gfx-rs/metal-rs/issues/58
• Later: https://github.com/gfx-rs/metal-rs/issues/282
• Later: https://github.com/gfx-rs/metal-rs/issues/284